1. Field of the Invention
The present invention relates to an electrostatic electrophotography apparatus of the electrophotography method for forming an electrostatic latent image on an image carrier, and developing the electrostatic latent image with toner, thereby copying or printing an image and, more particularly, to an electrostatic electrophotography apparatus capable of stably performing high-quality development by using an insulating one-component developing agent as a developing agent.
2. Description of the Related Art
An arrangement of an electrostatic electrophotography apparatus serving as a printer of the electrophotography method is shown in FIG. 23.
The arrangement shown in FIG. 23 is an electrostatic electrophotography apparatus of a dry type called ionography. FIG. 23 is a view for explaining the outline of this electrostatic electrophotography apparatus.
As shown in FIG. 23, a drum-shaped image carrier 1 made of a dielectric material is used. A latent image forming unit 2, e.g., an ion cartridge is arranged for forming an electrostatic latent image on the image carrier 1. A developing unit 3 supplies toner to the image carrier 1. An electrostatic latent image formed on the image carrier 1 by the latent image forming unit 2 is developed as a toner image by the developing unit 3.
A transfer unit 4 generates positive or negative corona ions. A sheet 5 is supplied to the position of the image carrier 1. The transfer unit 4 emits corona ions toward the image carrier 1 via the sheet 5. The sheet 5 is charged by the corona ions. By this charging, the toner image on the image carrier 1 is attracted to and transferred on the sheet 5. The toner image transferred on the sheet 5 is not fixed yet. The recording sheet 5 having the non-fixed toner image thereon is conveyed to a fixing unit 7 by a paper convey unit 6. The fixing unit 7 fixes the non-fixed toner image on the sheet 5.
The toner not transferred to the sheet 5 and remaining on the image carrier 1 is scraped by a cleaning unit 8. The electrostatic latent image remaining on the image carrier 1 is erased by an eraser 9. The toner scraped by the cleaning unit 8 is collected by a collecting unit 13.
A paper feed unit 10 feeds the sheets 5 one by one in order to transfer toner images. The paper feed unit 10 has a sheet cassette 10a and a pickup roller 10b. The sheet cassette 10a stores cut sheets (standard sheets) in a stacked manner therein. The pickup roller 10b picks up the sheets 5 one by one from the recording sheet cassette 10a. The sheet 5 fed by the paper feed unit 10 is guided to the position of the transfer unit 4 by a paper guide member 11. A guide member 14 is used for guiding the sheet 5 from the position of the transfer unit 4 to the fixing unit 7.
The image carrier 1 has a drum shape, and the latent image forming unit 2, the developing unit 3, the transfer unit 4, the toner collecting unit 13, the cleaning unit 8, and the eraser 9 are arranged at predetermined positions around the image carrier 1. As the image carrier 1 is rotated, the circumferential surface of the image carrier 1 sequentially passes the positions of these units.
In the apparatus shown in FIG. 23, when printing is performed, the image carrier 1 is rotated at a predetermined speed. The electrostatic latent image of an image to be printed is formed on the circumferential surface of the image carrier 1 by the latent image forming unit 2. The toner is supplied to the surface of the image carrier 1 by the developing unit 3, so that the toner attaches to the electrostatic latent image forming portion on the image carrier 1. Hence, the electrostatic latent image forms a toner image. A sheet 5 is fed to the position of the transfer unit 4 by the paper feed unit 10, and the sheet 5 is moved at the same speed as the peripheral speed of the image carrier 1.
The toner image is transferred to the recording sheet 5 by the transfer unit 4 and sent to the fixing unit 7 by the paper convey unit 6. The toner image on the sheet 5 is fixed by being heated by the fixing unit 7.
In this manner, a latent image is developed by a toner serving as a developing agent, transferred to a sheet 5, and thermally fixed, so that the latent image is printed. Electrostatic electrophotography apparatuses include, in addition to this ionography printer (IDP), a laser beam printer (LBP) for forming a latent image on the image carrier 1 with a laser beam.
In an electrostatic electrophotography apparatus, e.g., an IDP or a LBP, the developing process is performed by applying toner by a developing unit usually employing a one- or two-component developing agent.
Of one- and two-component developing agents, a magnetic one-component toner is often used because of its easy maintenance. A magnetic one-component toner developing method is a method using a developing agent made of only an insulating toner containing magnetic particles. Accordingly, this developing agent does not contain a magnetic carrier.
The toner particles used in the magnetic one-component toner developing method have an average particle diameter of about 10 .mu.m. One toner particle contains about 30 to 60 wt. % of magnetic particles, e.g., magnetite. A typical structure of the developing unit 3 using the magnetic one-component toner is as shown in FIGS. 24 and 25.
More specifically, as shown in FIGS. 24 and 25, the developing unit 3 comprises a columnar magnet roller 21 and a cylindrical sleeve 22 made of, e.g., stainless steel and arranged outside the magnet roller 21. When either or both of the magnet roller 21 and the sleeve 22 are rotated, charging and conveyance of the toner is performed. At this time, the toner on the sleeve 22 is uniformed to a thin layer by a blade 23 and transported to the image carrier 1, thereby developing the electrostatic latent image on the image carrier 1.
When the section of the columnar magnet roller 21 is observed, for example, a plurality of magnetic poles are alternately formed in the magnet roller 21 equiangularly about the central axis of the column as the center. The magnet roller 21 has this sectional structure throughout its length.
The sleeve 22 stores the magnet roller 21 therein. The sleeve 22 is arranged to be close to the image carrier 1 such that the central axes of the sleeve 22 and the image carrier 1 are parallel to each other. The sleeve 22 is rotated at the same peripheral speed as that of the image carrier 1.
In the arrangement shown in FIG. 24, the magnet roller 21 is stationary and stopped at a predetermined position in the sleeve 22. In the arrangement shown in FIG. 25, the magnet roller 21 is rotatable, and is rotated in the sleeve 22 in a direction opposite to the rotating direction of the sleeve 22.
The sleeve 22 incorporating the magnet roller 21 is arranged in a box body 24 of the developing unit 3. The space in which the sleeve 22 is arranged in the box body 24 is not a closed independent space, but a portion of the sleeve 22 opposing the image carrier 1 is moved close to the image carrier 1 while it is exposed in the box body 24.
The blade 23 is arranged above the sleeve 22. A magnetic toner serving as the developing agent is filled in the box body 24, and is attracted to the surface of the sleeve 22 by the magnetism of the magnet roller 21 of the sleeve 22. As the sleeve 22 is rotated, the magnetic toner attracted to the surface of the sleeve 22 is conveyed to the image carrier 1 side. At this time, the blade 23 uniforms the thickness of the toner layer on the sleeve 22 to form a thin layer throughout the entire length of the sleeve 22.
As the sleeve 22 is rotated, the magnetic poles of the magnet roller 21 in the sleeve 22 are moved in a direction relatively opposite to the moving direction of the sleeve 22. Thus, due to the influence of the movement of the magnetic poles of the magnet roller 21, the magnetic toner attaching to the surface of the sleeve 22 is rotated and agitated on the surface of the sleeve 22. Because of friction of the toner particles accompanying this agitation, the magnetic toner is charged.
If the polarity of the charges and the polarity of the latent image formed on the image carrier 1 are opposite, a toner image appears on the image carrier 1 since the toner is attracted by the latent image, thereby performing development.
The insulating one-component developing method is a method described so far. This method is advantageous in terms of easy maintenance and little contamination of the machine itself caused by the toner.
These advantages are obtained partly because the insulating one-component developing method uses only a magnetic toner as a developing unit. That is, the developing agent does not contain at all a carrier serving as the magnetic particles for charging the toner by agitation, unlike in the two-component developing agent. Since the developing agent does not contain a carrier at all, the concentration of the toner need not be detected. Since this method is free from the problem of degradation in performance of the carrier, the developing agent need not be exchanged, thereby simplifying maintenance.
Since a heavy carrier is not contained, the developing agent is lightweight, so that it can be handled easily, leading to an easy maintenance.
One of other advantages is obtained because a magnetic toner is used. If the toner used is magnetic one, the toner scatters less than non-magnetic toner in the machine, leading to a long maintenance cycle. If the interior of the machine is not soiled with the scattered toner, the service life of the machine is prolonged.
This developing method, however, has following problems.
If the potential of the latent image is low, the image quality of the toner image obtained by development is low. For example, if the potential of the latent image is 600 V or less in absolute value, especially if the electrostatic latent image has a low potential of .+-.300 V, the image quality of the toner image obtained by development is low. This is because the magnetic toner is used and a carrier is not contained. Since the magnetic toner is used, the image quality can be easily influenced by the magnetism of the magnet roller when compared to the image quality obtained by using the two-component developing agent. Since a carrier is not contained, a sufficient effect of agitation cannot be obtained, and the charge amount of the toner is decreased. If the charge amount of the toner is low and the magnetic toner is attracted by the magnetism of the magnet roller, the attraction force of the toner with respect to the charges of the electrostatic latent image is further decreased, and the toner image cannot be developed easily.
For these reasons, in order to improve the developing capability for a low-potential latent image, the content of a magnetic powder per toner particle is decreased, or the addition amount of an antistatic agent is increased. These countermeasures, however, lead to other problems.
First, these countermeasures cause fog to soil an image. Second, the image lacks a long-term stability.
Regarding the first problem of fog, generally, the content of the magnetic powder is inversely proportional to the obtained density, and the addition amount of the antistatic agent is inversely proportional to the fog level. Also, when the density of an image to be obtained is decreased, fog is decreased. Accordingly, in order to obtain an optimal state, the image density and the fog state must be set to intermediate levels. As a result, only a marginally satisfactory image quality as a compromise between the density and fog can be obtained.
Regarding the second problem of the lack of long-term stability in image formation, when the content of the magnetic powder is decreased or the addition amount of the antistatic agent is increased, charging of the toner becomes unstable during use, and the toner is partly charged in the opposite polarity. Then, a lack in long-term stability is caused.
If charging is unstable, as the number of times of printing operations is increased, the image density is undesirably decreased. This leads to a phenomenon in which even if the toner amount is sufficient, the density of the printed image is insufficient. Although this phenomenon is temporarily recovered by replenishing the toner, the image density level is quickly decreased to that obtained before replenishment. In this manner, the density level lacks stability.
The degree of degradation in image quality progresses as the print count increases. Therefore, as the print count increases after exchanging the developing agent, the image quality remarkably deteriorates. Then, even if the toner amount is sufficient, the developing agent must be exchanged. This is a bothersome problem that inevitably occurs, by and large, in the insulating one-component developing agent. This means that the maintenance frequency of the printer must be increased, thereby hindering realization of a printer with expendable supplies (e.g., a developing agent) having long exchange cycles, i.e., a printer requiring less management and maintenance.
The third problem is image non-uniformity.
When the insulating one-component developing agent is used, if an image having a high latent image potential level and a wide area, e.g., a solid black image, is developed, charges on the drum (image carrier 1) are undesirably shifted to the toner particles to inverse the polarity of the charges of the toner. As a result, the toner undesirably attaches to the sleeve 22 of the developing unit.
When the toner attaches to the sleeve 22, the image density of this portion lowers if the corresponding image portion is a solid black image portion and a fog occurs if the corresponding image portion is a white image portion, leading to an image having a remarkably low image quality with many non-uniform density portions. This phenomenon occurs easily when the content of the magnetic powder is decreased.